attempt for new tests(which compile but do not yet even fail)

library/src/main/java/org/mustangproject/Invoice.java

@@ -33,6 +33,7 @@ public class Invoice implements IExportableTransaction {
 	protected BigDecimal totalPrepaidAmount = null;
 	protected IZUGFeRDExportableContact ownContact = null, recipient = null, deliveryAddress = null;
 	protected ArrayList<IZUGFeRDExportableItem> ZFItems = null;
+	protected String contractReferencedDocument = null;
 
 	protected ArrayList<IZUGFeRDAllowanceCharge> Allowances = new ArrayList<IZUGFeRDAllowanceCharge>(),
 			Charges = new ArrayList<IZUGFeRDAllowanceCharge>(), LogisticsServiceCharges = new ArrayList<IZUGFeRDAllowanceCharge>();
@@ -50,6 +51,8 @@ public class Invoice implements IExportableTransaction {
 		return documentName;
 	}
 
+	public String getContractReferencedDocument() { return contractReferencedDocument; }
+
 	public Invoice setDocumentName(String documentName) {
 		this.documentName = documentName;
 		return this;
@@ -419,4 +422,9 @@ public class Invoice implements IExportableTransaction {
 		Allowances.add(izac);
 		return this;
 	}
+
+	public Invoice setContractReferencedDocument(String s) {
+		contractReferencedDocument=s;
+		return this;
+	}
 }

library/src/main/java/org/mustangproject/ZUGFeRD/IExportableTransaction.java

@@ -95,6 +95,10 @@ public interface IExportableTransaction {
 		return null;
 	}
 
+	default String getContractReferencedDocument() {
+		return null;
+	}
+
 
 	/**
 	 * who processed the order